1. Field of the Invention
This invention pertains generally to fluid stream separation schemes and methods for producing metal-organic frameworks, and more particularly to the production and use of a metal-organic framework with the formula Fe2(BDP)3 which can efficiently separate the isomers of C5-C7 alkanes.
2. Description of Related Art
The separation of molecules that are structurally-similar and chemically-similar from a product stream can be particularly challenging. Conventional separation schemes use solid adsorbents such as zeolites and activated carbons for key gas storage and molecular separation applications. However, these separations are often dependent on cryogenic or distillation steps that are both extremely energy-intensive strategies and often inefficient resulting in the production of low purity separations. For example, the efficient separation of alkane isomers by adsorption is especially challenging because the molecules are chemically inert and have similar polarizabilities, leaving shape as the main handle available for their differentiation. This alkane separation is critical to the production of gasoline, which is composed of approximately ten percent pentanes and hexanes.
For example, high-octane gasolines are more expensive than regular gasolines because of the cost and difficulty of separating the right type of molecules from crude petroleum. The creation of premium fuels requires the refinery to boil the petroleum at precise temperatures to separate out alkane isomers with only slightly different boiling points making the overall process both challenging and costly.
Hexanes of formula C6H14 are generated in large quantities through a catalytic isomerization reaction that results in a thermodynamically-controlled product stream composed of 10% to 30% of each of five different isomers. The product stream from the isomerization reactor, that commonly uses the zeolite MOR as a catalyst, consists of an equilibrium distribution of unreacted n-hexane (nC6), along with its mono-branched isomers 2-methylpentane (2MP), 3-methylpentane (3MP) and di-branched isomers 2,2-dimethylbutane (22DMB) and 2,3-dimethylbutane (23DMB).
In current industrial practice, the linear nC6 is separated from the branched isomers in an adsorption separation step that relies on molecular sieving. The typical adsorbent that is used is LTA-5A that consists of cages separated by 4.1 Å sized windows. The windows only allow the diffusion and adsorption of the linear nC6 isomer, and the branched isomers are rejected and removed as product. The unreacted nC6 is then recycled back to the isomerization reactor.
The worth of a particular isomer as a component in the gasoline pool is related to its research octane number (RON), which is highest for the di-branched hexanes 2,3-dimethylbutane and 2,2-dimethylbutane, that have values of 105 and 94, respectively. The RON values for the mono-branched isomers 2-methylpentane and 3-methylpentane are significantly lower, at 74 and 75, respectively, whereas the value for linear n-hexane is only 30. To achieve higher octane number fuel blends, current processes sieve n-hexane using zeolites, generating a mixture of the other four isomers with a final RON value of nearly 83, while returning n-hexane to the isomerization reactor. Additionally, some separation processes achieve higher-grade mixtures by subsequently distilling the mono-branched isomers away from the valuable dimethylbutane products. At the present time, approximately two million barrels of pentanes and hexanes are processed daily.
Therefore, di-branched isomers are preferred products for incorporation into the high-octane gasoline pool. An improved process would require the recycle of both linear and mono-branched isomers to the reactor. Typically, in such a processing scheme the aim would be to produce a product stream from the separation step with a RON value of 92. The separation of 22DMB and 23DMB from the remaining isomers is a difficult task because it requires distinguishing molecules on the degree of branching.
Accordingly, there is an need for an improved hexane separation process that selectively isolates the most valuable products, 2,3-dimethylbutane and 2,2-dimethylbutane, while returning the less valuable mono-branched isomers to the isomerization reactor along with n-hexane.
There is also a need for a system that performs this separation at or near the isomerization temperature thereby saving a great deal of energy in the production of high-quality gasoline. Furthermore, such a scheme would potentially benefit public health, since it could reduce the need to use toxic aromatics that are added to boost the octane number of gasoline.
The present invention satisfies these needs as well as others and is generally an improvement over the art.